Sol-gel methods make possible the preparation of very pure and homogeneous glasses of many compositions at relatively low temperatures. While almost any desired glass composition can be prepared by the alkoxide sol-gel process, it is at best very difficult to produce a relatively large glass body by the alkoxide process. For a review of multicomponent glasses made by the alkoxide processes see, for instance, E. M. Rabinovich, Journal of Materials Science, Vol. 20, pp. 4259-4280; see also I. M. Thomas, "Sol-Gel Technology for Thin Films, Fibers, Preforms, Electronics and Specialty Shapes", L. C. Klein, editor, Noyes Publications, pp. 1-15.
On the other hand, the particulate sol-gel process has made it possible to prepare quite large silica glass bodies, but it has not been possible to prepare other than at most lightly doped large silica bodies by the particulate process. For a review of the particulate process see, for instance E. M. Rabinovich, in "Sol-Gel Technology for Thin Films, Fibers, Preforms, Electronics, and Specialty Shapes", L. C. Klein, editor, Noyes Publications, 1988, pp. 260-294.
Starting at p. 274, the above cited publication also discusses a combined alkoxide-particulate method for making pure silica glass bodies. The method involves forming SiO.sub.2 particles by hydrolyzing TEOS (tetraethyl orthosilicate) mixed with ethanol, forming a sol by re-dispersing the particles in water, gelling the sol, and drying and sintering the thus formed gel body. The author also speculates that it may be easier to dope alkoxides with foreign ions that it is to so dope fumed silica. See also U.S. Pat. No. 4,605,428, which also discloses a combined alkoxide-particulate method.
U.S. Pat. No. 4,666,247 discloses optical fiber that comprises, in addition to silica, a "modifier" chosen from a group of elements that comprises Li, Na, K, Rb, and Cs, and the rare earths, and a "homogenizer" chosen from a group of elements that comprises Al. The modifier- and homogenizer-containing glass is formed by vapor deposition.
In many areas of technology it would be highly desirable to have available relatively large silica-based glass bodies of compositions not readily made by conventional processes. Exemplary of such glass compositions are low-alkali silicate glasses. Such glasses are difficult to prepare by conventional melting due to their high melting temperatures and the attendant volatility of the alkali components. Small laboratory specimens of some low alkali binary glasses were prepared by the melt process as early as 1932, and some of their properties measured. See, for instance, G. W. Morey et al., Journal of the Optical Society of America, Vol. 22(11), p. 632. See also N. L. Laberge et al., Journal of the American Ceramic Society, Vol. 56(1), p. 506; and J. Schroeder et al., ibid, p. 510. However, due to the difficulty of preparation, low-alkali silicate glasses (typically no more than about 12 mole % alkali oxide) have not yet become commercial.
It has recently been discovered (see U.S. patent application Ser. No. 790,836, filed Nov. 12, 1991 for M. E. Lines) that some low-alkali silicate glasses can have very low Rayleigh scattering and thus are of interest for optical fiber applications. This application discloses a method that advantageously can be used to produce relatively large silica-based glass bodies, including low-alkali silicate glass bodies.